Chapter 4 Writing Classes. 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes...

Chapter 4

Writing Classes

© 2004 Pearson Addison-Wesley. All rights reserved 4-2

Writing Classes• We've been using predefined classes. Now we will

learn to write our own classes to define objects

• Chapter 4 focuses on: class definitions instance data encapsulation and Java modifiers method declaration and parameter passing constructors graphical objects events and listeners buttons and text fields


OutlineAnatomy of a Class

Encapsulation

Anatomy of a Method


Writing Classes• The programs we’ve written in previous examples

have used classes defined in the Java standard class library

• Now we will begin to design programs that rely on classes that we write ourselves

• The class that contains the main method is just the starting point of a program

• True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality


Classes and Objects• Recall from our overview of objects in Chapter 1 that an

object has state and behavior

• Consider a six-sided die (singular of dice) It’s state can be defined as which face is showing

It’s primary behavior is that it can be rolled

• We can represent a die in software by designing a class called Die that models this state and behavior The class serves as the blueprint for a die object

• We can then instantiate as many die objects as we need for any particular program. Each die might have a different ‘state’ (value of its attribute, face) but the same ‘behaviors’ (can be rolled – a method)


Classes• A class can contain data declarations and method

declarations

int size, weight;char category;

Data declarations

Method declarations

Usually call these ‘attributes’ or ‘properties’Each of these has a ‘state’ that is, its ‘value’ at any particular instance in time.

These are the ‘behaviors’ or ‘methods’ or‘services’ objects of this class can provide.e.g. rollDie(); might be such a behavior.

All the ‘functionalities’, that is, the ‘actions’ that can take place on the attributes are included here and here alone.


Classes• The values of the data define the state of an object

created from the class

• The functionality of the methods define the behaviors of the object

• For our Die class, we might declare an integer that represents the current value showing on the face

• One of the methods would “roll” the die by setting that value to a random number between one and six


Classes• We’ll want to design the Die class with other data

and methods to make it a versatile and reusable resource

• Any given program will not necessarily use all aspects of a given class

• Let’s look at RollingDice.java (page 157)

• And Die.java (page 158).


• //********************************************************************• // RollingDice.java Author: Lewis/Loftus• //• // Demonstrates the creation and use of a user-defined class.• //********************************************************************

• public class RollingDice• {• //-----------------------------------------------------------------• // Creates two Die objects and rolls them several times.• //-----------------------------------------------------------------• public static void main (String[] args)• {• Die die1, die2;• int sum;

• die1 = new Die();• die2 = new Die();

• die1.roll();• die2.roll();• System.out.println ("Die One: " + die1 + ", Die Two: " + die2);

• die1.roll();• die2.setFaceValue(4);• System.out.println ("Die One: " + die1 + ", Die Two: " + die2);

• sum = die1.getFaceValue() + die2.getFaceValue();• System.out.println ("Sum: " + sum);

• sum = die1.roll() + die2.roll();• System.out.println ("Die One: " + die1 + ", Die Two: " + die2);• System.out.println ("New sum: " + sum);• }• }


• // Die.java // Represents one die (singular of dice) with faces showing values // between 1 and 6.• // Document as in the book – not here. This was done for space considerations!!• public class Die • {• private final int MAX = 6; // maximum face value• private int faceValue; // current value showing on the die• // Constructor: Sets the initial face value• public Die().• {• faceValue = 1;• }

• public int roll() // Rolls the die and returns the result.• {• faceValue = (int)(Math.random() * MAX) + 1;• return faceValue;• }

• public void setFaceValue (int value) // Face value mutator.• {• faceValue = value;• // get and set are standard.• public int getFaceValue() // Face value accessor• {• return faceValue;• }

• public String toString() // Returns a string representation of this die.• {• String result = Integer.toString(faceValue);• return result;• }• } // end class Die.


The Die Class• The Die class contains two data values

a constant MAX that represents the maximum face value

an integer faceValue that represents the current face value

• The roll method uses the random method of the Math class to determine a new face value

• There are also methods to explicitly set and retrieve the current face value at any time


The toString Method• All classes that represent objects should define a toString method

• The toString method returns a character string that represents the object in some way

• It is called automatically when an object is concatenated to a string or when it is passed to the println method


Constructors• As mentioned previously, a constructor is a

special method that is used to set up an object when it is initially created

• A constructor has the same name as the class

• The Die constructor is used to set the initial face value of each new die object to one

• We examine constructors in more detail later in this chapter


Data Scope• The scope of data is the area in a program in

which that data can be referenced (used)

• Data declared at the class level can be referenced by all methods in that class. Called Instance Data.

• Data declared within a method can be used only in that method

• Data declared within a method is called local data

• In the Die class, the variable result is declared inside the toString method -- it is local to that method and cannot be referenced anywhere else


Instance Data• The faceValue variable in the Die class is called

instance data because each instance (object) that is created has its own version of it

• A class declares the type of the data, but it does not reserve any memory space for it

• Every time a Die object is created, a new faceValue variable is created as well

• The objects of a class share the method definitions, but each object has its own data space

• That's the only way two objects can have different states


Instance Data• We can depict the two Die objects from the RollingDice program as follows:

die1 5faceValue

die2 2faceValue

Each object maintains its own faceValue variable, and thus its own state


UML Diagrams• UML stands for the Unified Modeling Language

• UML diagrams show relationships among classes and objects

• A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods)

• Lines between classes represent associations

• A dotted arrow shows that one class uses the other (calls its methods) (sends a message…)


UML Class Diagrams• A UML class diagram for the RollingDice

program:

RollingDice

main (args : String[]) : void

DiefaceValue : introll() : intsetFaceValue (int value) : voidgetFaceValue() : inttoString() : String



Encapsulation

Anatomy of a Method

Graphical Objects

Graphical User Interfaces

Buttons and Text Fields


Encapsulation• We can take one of two views of an object:

internal - the details of the variables and methods of the class that defines it

external - the services that an object provides and how the object interacts with the rest of the system

• From the external view, an object is an encapsulated entity, providing a set of specific services

• These services define the interface to the object


UML Class Diagrams• A UML class diagram for the RollingDice

program:

RollingDice

main (args : String[]) : void

DiefaceValue : introll() : intsetFaceValue (int value) : voidgetFaceValue() : inttoString() : String

• The ‘services’ provided by objects of type, Die, are roll(), setFaceValue(int), getFaceValue(), and toString(). • Messages to invoke any of those services might appear like, die1.getFaceValue(); or rolledNumber = die1.roll(); Okay???


Encapsulation• One object (called the client) may use another

object for the services it provides

• The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished

• Any changes to the object's state (its variables) should be made by that object's methods

• We should make it difficult, if not impossible, for a client to access an object’s variables directly

• That is, an object should be self-governing


Encapsulation• An encapsulated object can be thought of as a

black box -- its inner workings are hidden from the client

• The client invokes the interface methods of the object, which manages the instance data

Methods

Data

Client


Visibility Modifiers• In Java, we accomplish encapsulation through the

appropriate use of visibility modifiers

• A modifier is a Java reserved word that specifies particular characteristics of a method or data

• We've used the final modifier to define constants

• Java has three visibility modifiers: public, protected, and private

• The protected modifier involves inheritance, which we will discuss later


Visibility Modifiers• Members of a class that are declared with public

visibility can be referenced anywhere (via the object name.Membername.

• Members of a class that are declared with private visibility can be referenced only within that class

• Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package (package scope!)

• An overview of all Java modifiers is presented in Appendix E


Visibility Modifiers• Public variables violate encapsulation because

they allow the client to “reach in” and modify the values directly

• Therefore instance variables should not be declared with public visibility. They are usually always ‘private.’ (Will talk about ‘protected’ later)

• It is acceptable to give a constant public visibility, which allows it to be used outside of the class

• Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed


Visibility Modifiers• Methods that provide the object's services are

declared with public visibility so that they can be invoked by clients

• Public methods are also called service methods

• A method created simply to assist a service method is called a support method

• Since a support method is not intended to be called by a client, it should not be declared with public visibility

• Example: Have method to computePay(….). But the code requires help of computePerDiem(…). So, computePay would invoke (private!) computePerDiem method.


Visibility Modifiers

public private

Variables

Methods Provide servicesto clients

Support othermethods in the

class

Enforceencapsulation

Violateencapsulation


Accessors and Mutators• Because instance data is private, a class usually

provides services to access and modify data values

• An accessor method returns the current value of a variable

• A mutator method changes the value of a variable

• The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value

• They are sometimes called “getters” and “setters”


Mutator Restrictions• The use of mutators gives the class designer the

ability to restrict a client’s options to modify an object’s state

• A mutator is often designed so that the values of variables can be set only within particular limits

• For example, the setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX)

• We’ll see in Chapter 5 how such restrictions can be implemented



Encapsulation

Anatomy of a Method

Graphical Objects

Graphical User Interfaces

Buttons and Text Fields


Method Declarations• Let’s now examine method declarations in more

detail

• A method declaration specifies the code that will be executed when the method is invoked (called)

• When a method is invoked, the flow of control jumps to the method and executes its code

• When complete, the flow returns to the place where the method was called and continues

• The invocation may or may not return a value, depending on how the method is defined


myMethod();

myMethodcompute

Method Control Flow• If the called method is in the same class, only the

method name is needed

Here, myMethod is found in the same object. Only the method name is needed to invoke the method to execute.


doIt

helpMe

helpMe();

obj.doIt();

main

Method Control Flow• The called method is often part of another class or

object

Here, main() calls a method IN A DIFFERENT OBJECT. So it must call the method via the standard call: object.method. Here, that is obj.doIt(). Within doIt(), there is a call to another method in the same object. Only the method name is therefore necessary.


Method Header• A method declaration begins with a method header

char calc (int num1, int num2, String message)

methodname

returntype

parameter list

The parameter list specifies the typeand name of each parameter

The name of a parameter in the methoddeclaration is called a formal parameter


Method Body• The method header is followed by the method body

char calc (int num1, int num2, String message){ int sum = num1 + num2; char result = message.charAt (sum);

return result;}

The return expressionmust be consistent withthe return type

sum and resultare local data

They are created each time the method is called, and are destroyed when it finishes executing


The return Statement• The return type of a method indicates the type of

value that the method sends back to the calling location

• A method that does not return a value has a void return type

• A return statement specifies the value that will be returned

return expression;

• Its expression must conform to the return type


Parameters• When a method is called, the values of the actual parameters

(oftentimes called ‘arguments) in the invocation are copied into the formal parameters in the method header

char calc (int num1, int num2, String message){ int sum; // sum is a local variable!! sum = num1 + num2; char result = message.charAt (sum);

return result;}

ch = obj.calc (25, count, "Hello");

Function call

Value ‘returned’ is assigned to ch.


Local Data• As we’ve seen, local variables can be declared

inside a method (See int sum; last slide)

• The formal parameters of a method create automatic local variables when the method is invoked

• When the method finishes, all local variables are destroyed (including the formal parameters)

• Keep in mind that instance variables, declared at the class level, exists as long as the object exists. But instance variables are declared outside of any methods.


Bank Account Example• Let’s look at another example that demonstrates the

implementation details of classes and methods

• We’ll represent a bank account by a class named Account

• It’s state can include the account number, the

current balance, and the

name of the owner

• An account’s behaviors (or services) include deposits and withdrawals, and adding interest


Bank Account Example• Note: the program canNOT start here!

• The next figure is a class!!!

• All ‘applications’ must start with a main routine in some kind of ‘driver.’ (More ahead).

• Let’s look at the class definition of Account!


// Account.java Author: Lewis/Loftus// Represents a bank account with basic services such as deposit and withdraw.//********************************************************************import java.text.NumberFormat;public class Account.{ private final double RATE = 0.035; // interest rate of 3.5% private long acctNumber; private double balance; private String name;

// Sets up the account by defining its owner, account number and initial balance. public Account (String owner, long account, double initial) { name = owner; acctNumber = account; balance = initial; }

// Deposits the specified amount into the account. Returns the new balance. public double deposit (double amount) { balance = balance + amount; return balance; }

// Withdraws the specified amount from the account and applies the fee. Returns the new balance. public double withdraw (double amount, double fee) { balance = balance - amount - fee; return balance; }

Instance data. EACH object of type Account will get its own copy of these variables.Note: all ‘private.’ (means only methods of these objects can accessthese variables.

Note: the Constructor.Same name as class.Initializes attributes, and sets up initial ‘state’ of an object.

Note: all methods public (here).Note: method names, formal params, and return types, if any.


//----------------------------------------------------------------- // Adds interest to the account and returns the new balance. //----------------------------------------------------------------- public double addInterest () { balance += (balance * RATE); return balance; }

//----------------------------------------------------------------- // Returns the current balance of the account. //----------------------------------------------------------------- public double getBalance () { return balance; }

//----------------------------------------------------------------- // Returns a one-line description of the account as a string. //----------------------------------------------------------------- public String toString () { NumberFormat fmt = NumberFormat.getCurrencyInstance();

return (acctNumber + "\t" + name + "\t" + fmt.format(balance)); }}


Driver Programs• A driver program drives the use of other, more

interesting parts of a program

• Driver programs are often used to test other parts of the software

• The Transactions class contains a main method that drives the use of the Account class, exercising its services

• See Transactions.java (page 172)


• // Transactions.java Author: Lewis/Loftus• // Demonstrates the creation and use of multiple Account objects.• public class Transactions• {• // Creates some bank accounts and requests various services.• public static void main (String[] args)• {• Account acct1 = new Account ("Ted Murphy", 72354, 102.56);• Account acct2 = new Account ("Jane Smith", 69713, 40.00);• Account acct3 = new Account ("Edward Demsey", 93757, 759.32);

• acct1.deposit (25.85);

• double smithBalance; smithBalance = acct2.deposit (500.00);

• System.out.println ("Smith balance after deposit: " + smithBalance);

• System.out.println ("Murphy balance after withdrawal: " + acct2.withdraw (430.75, 1.50));

• acct1.addInterest();• acct2.addInterest();• acct3.addInterest();

• System.out.println ();• System.out.println (acct1);• System.out.println (acct2);• System.out.println (acct3);• }• }


Bank Account Example

acct1 72354acctNumber

102.56balance

name “Ted Murphy”

acct2 69713acctNumber

40.00balance

name “Jane Smith”


Bank Account Example• There are some improvements that can be made to

the Account class

• Formal getters and setters could have been defined for all data

• The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive


Constructors Revisited• Note that a constructor has no return type

specified in the method header, not even void

A common error is to put a return type on a constructor, which makes it a “regular” method that happens to have the same name as the class

• The programmer does not have to define a constructor for a class

• Each class has a default constructor that accepts no parameters

Chapter 4 Writing Classes. 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes...

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